摘要:

Aminoacyl ethyl phosphates are functional analogues of aminoacyl adenylates, the activated intermediates formed by ribosomal enzymes from the reaction of amino acids with ATP. The aminoacyl ethyl phosphates are prepared by DCC-mediated coupling of a tetraalkylammonium salt of ethyl phosphate and anN-t-Boc-amino acid, followed by deprotection. The efficiency of the synthesis and the solubility properties of the product depend on the alkyl group in the ammonium counterion of the phosphate.N-Protected and free aminoacyl ethyl phosphates hydrolyze slowly (in neutral solution, 25 °C, ~100 h). These materials are potentially useful in biomimetic peptide synthesis, protein modification, and aminoacylation of RNA.Keywords: amino acids, acyl phosphates, aminoacyl phosphates, biomimetic aminoacylation.

ISSN:0008-4042    

DOI:10.1139/v96-268

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

A series of α-diazoketones,8,25,28,31, and34, have been synthesized and their reaction with ethyl vinyl ether examined under various reaction conditions. In the presence of metal salts (Rh2(OAc)4, Pd(OAc)2, CuCl) the ethoxy-dihydrofurans12,37,39,41, and43are produced. Sensitized irradiation of the α-diazoketone8afforded the dihydrofuran12plus cyclobutanone7, while direct photolysis of α-diazoketones8,25,28,31, and34gave the cyclobutanones7,38,40,42, and44, respectively. A sample of the cyclopropylketone45was isolated from the rhodium(II) acetate mediated reaction of34and its facile rearrangement to dihydrofuran43demonstrated. Collectively, these results indicate that the initial product from the reaction of an α-diazoketone with an electron-rich alkene such as ethyl vinyl ether is a cyclopropylketone. The donnor–acceptor substitution pattern of this intermediate results in spontaneous rearrangement to a dihydrofuran. Thus a direct dipolar cycloaddition mechanism is not involved when α-diazoketones react with enol ethers under metal-mediated conditions. Instead, these reactions follow a cyclopropanation rearrangement or, more accurately, cyclopropanation – ring opening – cyciization pathway.Keywords: diazoketone, rhodium acetate, dihydrofuran, cyclopropylketone, vinyl ether.

ISSN:0008-4042    

DOI:10.1139/v96-269

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Manganese and cadmium complexes of singly and doubly deprotonated 2,3-pyrazinedicarboxylic acid (pyzdcH2) have been prepared and studied. Crystal data: [H3O]2χ[Mn(pyzdc)2]x,1, monoclinic,C2/c(no. 15),a = 14.472(1) Å,b = 8.475(2) Å,c = 13.0771(9) Å, β = 114.753(4)°,Z = 4; [H3O]2x[Cd(pyzdc)2]x,2, monoclinic,C2/c(no. 15),a = 14.4857(9) Å,b = 8.502(1) Å,c = 14.9803(7) Å, β = 126.951(2)°,Z = 4; [Mn(pyzdc)(H2O)2]x•2xH2O,3, monoclinic,P21/c(no. 14),a = 6.5523(7) Å,b = 7.6702(7) Å,c = 20.5815(6) Å, β = 93.605(5)°,Z = 4; [Cd(pyzdc)(H2O)3]x•xH2O,4, monoclinic,Cc(no. 9),a = 6.088(1) Å,b = 11.5410(9) Å,c = 15.0624(7) Å, β = 96.532(8)°,Z = 4. The structures were solved by Patterson (1,4) or direct (2,3) methods and were refined by full-matrix least-squares procedures toR = 0.036, 0.025, 0.034, and 0.022 (Rw = 0.032, 0.025, 0.036, and 0.023) for 2825, 4961, 1672, and 2641 reflections withI ≥ 3σ(F2), respectively. The structures of1and2consist of extended anionic chains of metal ions that are linked by double-bridging pyzdc2−groups, with electrical neutrality maintained by the presence of H3O+cations in the lattice. In3double chains of manganese ions are cross-linked by pyzdc2−ligands forming a "ladder-like" motif. The structure of4consists of extended chains of cadmium ions in which single pyzdc2−groups bridge metal centres. Magnetic susceptibility studies indicate, at most, only weak antiferromagnetic exchange in1but moderately strong antiferromagnetic interactions in3, mediated by bridging carboxylate groups (J = −0.27 cm−1).Keywords: extended chains, manganese, cadmium, 2,3-pyrazinedicarboxylates, X-ray structures, antiferromagnetism.

ISSN:0008-4042    

DOI:10.1139/v96-270

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

In the course of a preliminary investigation on the relationships between the chemical structure of ryanoids and their affinity to the ryanodine binding site, we have isolated, from the plantRyaniaspeciosaVahl, four new members of this family of natural insecticidal compounds (ryanoids3,4,5, and6) and corrected the reported structure of a fifth one (ryanoid7). In addition, we have synthesized, from anhydroryanodol (10), new members of this family having fewer hydroxyl groups in ring A: cinnzeylanol (14) and cinnzeylanine (15), 2,3-dideoxy-2,3-dihydroryanodol (16), 2-deoxy-3-epiryanodol (18), and 2-deoxy-3-epiryanodine (19), 2-deoxyryanodol (20), and 2-deoxy-2-epiryanodol (21).Keywords: ryanoids synthesis, cinnzeylanine, 2-deoxyryanodols, 2-deoxy-3-epiryanodine.

ISSN:0008-4042    

DOI:10.1139/v96-271

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

A new method for the synthesis oftrans-2,6-disubstituted piperidine derivatives is described. The transformation of cyclic α-methoxycarbamates5and6affordstransketones17and18. The synthesis of diols1–4from17and18has shown that the structure proposed in the literature for the piperidine alkaloid andrachamine is incorrect. A reexamination of the original spectral data of this alkaloid suggested that it is ameso2,6-disubstituted piperidine derivative. Unambiguous syntheses of23and24and comparison with a sample of andrachamine have established that this alkaloid possesses structure23.Keywords:N-acyliminium, piperidine, alkaloid, andrachamine.

ISSN:0008-4042    

DOI:10.1139/v96-272

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The β-hydroxypiperidine alkaloids (±)-pseudoconhydrine, (±)-N-methylpseudoconhydrine, (−)-5-hydroxysedamine, and (+)-sedacryptine were synthesized. Successive functionalization of the piperidine ring via anodic methoxylation allowed the regio- and stereoselective introduction of the substituents. The α and α′ substituents were introduced by application of the sequence elimination–nucleophilic addition from 2- or 2,5-substituted 6-methoxycarbamates. Hydroboration – oxidation of enecarbamates, obtained by elimination of methanol from α-methoxycarbamates, allowed the introduction of the β-hydroxy function.Keywords: alkaloid, Sedum,N-acyliminium, enecarbamate.

ISSN:0008-4042    

DOI:10.1139/v96-273

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The polarographic method has been applied to the study of the mixed-coordinated system (Pb(II) – neutral valine – valinate ion, in aqueous medium,I = 1.0 mol L−1(NaClO4) and 25 ± 0.1 °C. The stabilization of the following complexes: [Pb(HV)]2+(β10 = 9.7), [Pb(HV)2]2+(β20 = 28), [Pb(HV)(V)]+(β11 = 3.3 × 104), [Pb(V)]+(β01 = 3.7 × 104), and [Pb(V)2] (β02 = 2.8 × 107) was observed.Keywords: polarography, Pb(II), valine, speciation.

ISSN:0008-4042    

DOI:10.1139/v96-274

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Dissolved organic carbon (DOC) was quantitatively isolated as five separate fractions from two Sudbury area lakes (Ontario, Canada) using the tandem XAD-8/XAD-4 resin technique. Fulvic acids made up ≈ 48% of the total, humic acids ≈ 8%, hydrophobic neutrals ≈ 8%, XAD-4 acids ≈ 11%, and XAD-4 acetonitrile extracts ≈ 13%. Hydrophobic:hydrophilic ratios were about 65:35. Solid-state CPMAS13C NMR indicated a primarily aliphatic nature in all fractions. Percent aromaticity was in the order humic acids > fulvic acids > XAD-4 acetonitrile extracts ≈ XAD-4 acids ≈ hydrophobic neutrals. Titration analysis showed that the13C NMR peaks of fulvic acids, humic acids, and XAD-4 acids from 160–190 ppm were primarily carboxyl in nature. The XAD-4 acids were the most acidic, suggesting that they may be important geochemical agents.13C NMR indicated that polysaccharides formed < 20% of any fraction. The two neutral fractions (hydrophobic neutrals and XAD-4 acetonitrile extracts) had relatively high N contents, possibly due to the acetonitrile used to extract them. However, the13C NMR spectra did not show significant quantities of acetonitrile. The elemental and structural composition covaries in the three acid fractions, such that if one acid is enriched or depleted, so are the others. This suggests that the humic and non-humic acids form a continuum in which all fractions are affected by the same formative processes. Compared to published data for other freshwater samples, the three acid fractions from the Sudbury area lakes have low or very low aromaticity. This may indicate that the acid and metal loadings in the Sudbury area have affected the structure of the organic acids, despite the circumneutral character of the study lakes. These are the first quantitative13C NMR results published for the hydrophobic neutrals and XAD-4 acetonitrile extracts, so comparisons are not possible.Keywords: aquatic DOC, organic acids,13C NMR, humic substances, non-hum

ISSN:0008-4042    

DOI:10.1139/v96-275

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The marine gastropod molluskTrimusculusperuvianuscollected on the coast of Central Chile yielded four new labdane metabolites. The structural elucidation of all compounds was accomplished by spectral analysis. One diterpenoid showed modest antimicrobial activity.Keywords:Trimusculusperuvianus, labdanes, marine mollusk.

ISSN:0008-4042    

DOI:10.1139/v96-276

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

G2 methodology has been used to characterize minima on both doublet and quartet potential energy surfaces of the PSi2radical system. We found that for states with doublet spin multiplicity the most stable isomer is the cyclic2A1. Linear isomers lie more than 24 kcal/mol above in energy. For the quartets the most stable state isomer is the cyclic4A2, and the most stable linear isomer, i.e., Si-Si-P(4∑−), lies 10.68 kcal/mol higher in energy. The structural features of the various isomers characterized have been rationalized in terms of the bonding features of the molecular orbitals involved.Keywords: ab initio, excited states, radical.

ISSN:0008-4042    

DOI:10.1139/v96-277

出版商:NRC Research Press    

年代:1996

数据来源: NRC